The foam cannons work very well on my cars. I haven't tried it on the tractor but would have the same concerns above about power washing around exposed electronics. Interestingly, although the foam cannon requires a pressure washer to function, the foam doesn't exit the nozzle with the same velocity as water through the wand that comes with the pressure washer. I think it might be OK to use it to soap up the outside of the machine but I would rinses carefully with a regular hose, not the pressure washer.
If you have a pressure washer, the foam cannons and the soap they use are fairly inexpensive on Amazon. I like the result on the cars...